In round balers of variable bale chamber design, the tension of the bale forming elements (e.g. belts) is typically defined by a hydraulic cylinder that interacts with a moving tension arm that carries a deflection roller abut which, in turn, the bale forming elements revolve (normally one or more belts or bar-chain conveyors). The pressure in the hydraulic cylinder is normally controlled by way of a pressure-relief valve whose opening pressure can be controlled electromagnetically. According to U.S. Pat. No. 4,611,535 a driving of the pressure-relief valve is provided in the sense of a reduction in the pressure in the hydraulic cylinder and thus the tension of the bale forming elements, when an excess torque is detected on the main drive shaft. In German Patent document DE 197 18 229 A1 it is proposed to drive the pressure-relief valve as a function of a desired bale density and bale size, while, in the non-energized state, the pressure-relief valve in European Patent Application EP 1 059 476 A2 is held in the closed state by a biased spring.
In the baler according to U.S. Pat. No. 4,611,635, the opening pressure of the pressure-relief valve can be adjusted by hand by means of plain washers that can be inserted between a spring and the armature of the electromagnet. In EP 1 059 476 A2, the opening pressure of the pressure-relief valve can be adjusted by hand by means of an adjustment screw changing the bias tension of a compression spring. This manual adjustability of the opening pressure of the pressure-relief valve is useful especially when the electronic controller of the pressure-relief valve is interfered with or has failed.
In both of the above-mentioned balers, a failure or interference in the electronic controller of the pressure-relief valve results in a relatively high opening pressure. Now, if the operator inadvertently sets the opening pressure of the pressure-relief valve even higher, then it is possible that a structurally specified maximum pressure in the hydraulic cylinder is exceeded, which could result in damage to the baler. This problem occurs to an elevated degree if the electromagnet (like in the embodiment according to FIG. 3 of U.S. Pat. No. 4,611,535) in the energized state increases the opening, and the error in the electronics leads to a permanent excitation of the electromagnet.